In recent years, in cellular mobile communication systems, with the combination of various kinds of information into multimedia, a large amount of data, such as voice data, still image data, and moving image data, is generally transmitted. A technique for achieving a high transmission rate using a high-frequency radio band has been actively studied in order to transmit a large amount of data.
When the high-frequency radio band is used, it is possible to expect a high transmission rate in a short distance. However, as the distance increases, attenuation due to a transmission distance increases. When a mobile communication system using the high-frequency radio band is operated in practice, the coverage of a radio communication base station device (hereinafter, simply referred to as a base station) is reduced. Therefore, it is necessary to set a large number of base stations. Since the setting cost of the base stations is great, there is a strong demand for a technique capable of providing a communication service using the high-frequency radio band while preventing an increase in the number of base stations.
For the demand, in order to expand the coverage of each base station, a relay transmission technique has been studied which sets radio communication relay station devices (hereinafter, simply referred to as relay stations) between a base station and a radio communication mobile station device (hereinafter, simply referred to as a mobile station) in multiple stages and performs communication between the base station and the mobile station through the relay stations, as shown in FIG. 1. When the relay transmission technique is used, the mobile station that is incapable of directly communicating with the base station can communicate with the base station through the relay stations.
However, in the multi-stage relay shown in FIG. 1, when the number of hops from a transmission source to a reception destination (the number of relay stages between the transmission source and the reception destination) increases, the amount of delay increases. Therefore, it is necessary to reduce the amount of delay. As a method of reducing the amount of delay, a method has been proposed which improves the reception quality of signals by each relay station to reduce the number of retransmissions (see Non-patent Literature 1).
Non-patent Literature 1 discloses a technique in which each relay station receives signals transmitted to other relay stations, stores the received signals in a buffer, and combines the signals at the maximum ratio, thereby improving the reception quality of signals by each relay station.
The relay method disclosed in Non-patent Literature 1 will be described using the arrangement of the relay stations shown in FIG. 1 as an example. A relay station 1, a relay station 2, a relay station 3, a relay station 4, and a base station 6 receive the signal transmitted from a mobile station 5. The relay stations 1 to 4 and the base station 6 store the received signals in the buffers. The relay station 2 combines the signal to be subsequently relayed from the relay station 1 with the signal that has been transmitted from the mobile station 5 and then stored in the buffer at the maximum ratio.
In this way, it is possible to improve the reception quality of the signal obtained by combining the signal from the mobile station 5 and the signal from the relay station 1 at the maximum ratio, as compared to reception quality when only the signal transmitted from the relay station 1 is received. Since the relay station 3 combines the signal from the mobile station 5, the signal from the relay station 1, and the signal from the relay station 2 at the maximum ratio, it is possible to further improve reception quality. As such, according to the method disclosed in Non-patent Literature 1, it is possible to improve the reception quality of signals by each relay station to reduce the number of retransmissions, thereby reducing the amount of delay.